Currently, plastic lunch boxes are widely used and gradually replacing other lunch boxes, because of its good sealing performance, being easy to clean, lightweight, beautiful appearance and etc. In the prior art, a lid of a lunch box covers its base, making the inside of the lunch box airtight, further leading to not only good heat preservation effect, but also effectively preventing food leakage. Because the air tightness directly affects the use of the lunch box, most of the lunch boxes in the prior art have good sealing performance and no other air channels. However, good performance of air tightness will bring certain disadvantages. For example, when a lunch box is used in low temperature environment, the contraction of the internal air makes the lunch box difficult to open under atmospheric pressure. The internal air of the lunch box will inflate when heating, easily resulting in lid deformation, even the lid bulging, and further causing food splash, etc. Meanwhile when the lunch box is heated, water vapor inside the lunch box can't be vented, and steam backflow phenomenon will appear, leading to difficulty in keeping the original taste of the food, inconvenience in using and potential safety hazard.